1. Field of the Invention:
The present invention relates to indirect extrusion apparatus.
2. Prior Art
The indirect extrusion apparatus has advantages in that friction between the billet and the container wall defining a billet receiving bore is practically eliminated, and there is almost no turbulent metal flow in the billet, thereby reducing extruding force. Further, surface layers of the billet can be extruded, since there is no need of leaving the container a thin sleeve of the billet which would otherwise be produced by a dummy block during direct extrusion for preventing surface friction and entrainment of surface oxides into the extruded product.
A major problem with indirect extrusion apparatus is the accumulation of skull on the bore wall in the container. The skull comprises accumulations of unwanted materials on the container wall and may include a layer of unextruded metal, dirt, lubricant, metal particles produced when the outer periphery of a die holder is held in frictional contact with the bore wall, and metal oxides formed on the billet during homogeneous heat treatment before extrusion. These foreign impurities, deposited between the metal billet and the container wall, find their way into the products during extrusion and appear as flaws on the finished products, thereby lessening their quality and possibly rendering them defective.
Various attempts have been made in the past to remove such skull in order to assure an efficient indirect extrusion operation. One typical effort has been a skim block shown in U.S. Pat. No. 3,184,944, issued May 25, 1965. The skim block has an outer surface shaped to provide a precisely controlled sliding fit between the skim block and the container bore. The skim block is moved through the container for removal of the accumulated materials after an indirect extrusion process is completed. With this type of arrangement, the block itself must be carefully machined to avoid damage or excessive wear to the bore wall, and skull skimming through the container increases down time of the extrusion press.
Another die holder that has been suggested for preventing skull formation is described in U.S. Pat. No. 3,522,721, issued Aug. 4, 1970. The separate die holder has an outer sealing surface that is configured also to provide a closely controlled sliding fit between the outer sealing surface and the container bore wall. The sealing surface involves careful machining. Further, the die holder allows the impurities to get into the extruded product as the billet is extruded.